1. Field of the Invention
The present invention relates to a mobile communications system and method that control a radio base station controllers sending and receiving a radio signal to and from mobile terminals.
2. Description of the Related Art
A radio access network (RAN) in a mobile communications system consists of radio base stations and base station control devices that control the base stations.
A RAN based on 3GPP (3rd Generation Partnership Projects) specification is called a UMTS Terrestrial Radio Access Network (UTRAN) and includes a number of radio base stations (Node B) 501–504 and a Radio Network Controller (RNC) 505 as shown in FIG. 1. RNC 505 handles signaling control information relating to radio access as well as handling user data such as audio and video. A Control Plane (C-plane) provides signaling transfer control, and a User Plane (U-plane) handles user data such as audio and video data.
In recent years, an approach has been proposed that divides the C-plane and the U-plane in an RNC and provides a number of U-planes for each C-plane. In the following description, a C-plane section is called an RCS (Radio Control Server) and a U-plane section is called a UPS (User Plane Server).
FIG. 2 is a block diagram showing a configuration of such a mobile communication system. Included in the system are UPS 609 provided for radio base stations (Node B) 601–604, UPS 610 provided for radio base stations (Node B) 605–608, and one RCS 611 provided for UPS 609 and UPS 610.
In the UTRAN described above, an RNC frame number (RFN), Node B frame number (BFN), a cell system frame number (SFN), and a connection frame number (CFN) are provided as timing information. FIG. 3 shows the relationships between these items of timing information. The items of timing information are defined as follows:    (1) The RNC uses RFN as timing reference and Node B uses BFN as its timing reference.    (2) RFN and BFN have a frame length of 10 ms and are controlled with a cycle length of 4096 frames, in the range from 0 to 4095 frames.    (3) The phase difference between RFN and BFN can be measured by a mechanism called Node Synchronization.    (4) The frame numbers indicating timing of a cell under the control of Node B is called SFN. The output timing of each cell is determined by an offset relative to BFN called Tcell.    (5) A user equipment (UE) such as a mobile phone that is in communication in a cell is also communicating with the UTRAN by using CFN. The timing of CFN is determined by a frame offset relative to SFN and a chip offset.
Details of the architecture described above are specified in 3GPP (3rd Generation Partnership Projects).
The Tcell, frame offset, chip offset for associating the items of timing information described above are values specified by the RNC and the phase difference between RFN and BFN can be measured by Node Synchronization. Accordingly, all of these values are held by the RNC.
The RNC must know the CFN defined for each UE in order to indicate the timing to the UE. For example, CFN is used to specify activation time at which the encription function of user information flying over the radio is activated and start time at which Radio link Reconfiguration is performed for configuring a radio link.
CFN of each UE can be obtained by calculating the offsets (Tcell, frame offset, chip offset, and the phase difference between RFN and BFN) described from RFN timing of the RNC. In particular, BFN timing can be obtained from RFN timing, SFN timing can be obtained from the BFN timing, and CFN timing of each UE can be obtained from the SFN timing.
In a configuration in which a single radio network controller controls a number of radio base stations as shown in FIG. 1, the radio network controller controls these timings. Therefore, the calculation of the CFN timing of each individual UE on the basis of RFN timing as described above can be calculated by the single radio network controller. In contrast, in a configuration in which a radio network controller is made up of an RCS and a UPS, RFN timing is controlled in the UPS as shown in FIG. 2. Therefore, the RCS must in some way know the RFN timing managed in the UPS. However, no protocol for how an RCS gets to know RFN timing managed in a UPS.